I saved my Win7 System paratition to an external drive. Then formatted and installed Win10 instead of Win7. Everything ok so far.
In order to get access to all the files on the saved folders I mounted this drive as S: Then started
icacls <folder> /reset /t /c /l
icacls s:\users /reset /t /c /l
for several folders on drive S: including s:\ProgramData, "s:\Program Files" and s:\Users. During the long lasting run of icacls with s:\Users I recognized that i could no longer start the system settings. I also couldn't use the search box. It didnt allow anything to enter. Win + R still worked.
I'm quite sure that this action also changed some access rights on drive C: (actual system). After doing a System Restore. Most things worked again as expected.
So please: What had happened? Why are permissions changed on drive c: although I used drive s:? Did it follow symlinks (I used /l to avoid this)?
And what can be done to change the permissions on a drive without changing any other drive?